Method and system for measuring optical properties of a medium using digital communication processing techniques

ABSTRACT

A system for measuring properties of a medium includes an electromagnetic generator for forming a CW carrier, a digital encoder for forming a digital message, and a modulator for modulating the CW carrier with the digital message to form a digitally modulated CW carrier. The medium provides a channel for propagating the digitally modulated CW carrier. The system further includes a receiver configured to receive the propagated, digitally modulated CW carrier, and a processor for measuring at least one property of the medium. The medium may be disposed within a gaseous atmosphere, a body of water, or a cell of a laboratory.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of application Ser. No. 10/959,368,filed Oct. 6, 2004, which itself claims priority of U.S. ProvisionalApplication Ser. No. 60/517,953, filed on Nov. 6, 2003, the contents ofeach are expressly incorporated herein by reference.

FIELD OF THE INVENTION

This invention relates to detecting and measuring optical properties ofa medium using light detection and ranging (LIDAR) techniques or laserdetection and ranging (LADAR) techniques. More specifically, thisinvention relates to measuring optical properties of a medium usingdigital communication processing techniques.

BACKGROUND OF THE INVENTION

Active remote sensing may be conceptualized as viewing radiationreflected and/or emitted from a certain location in one or morewavelength regions. Active remote sensing typically utilizes one or moresources of radiation (e.g., infrared, visible, or ultraviolet light) toilluminate a target area while measuring the reflected, scattered and/oremitted radiation at one or more receive detectors. Such remote sensingmay be performed from a moving platform or from a stationary location,each of which may be spatially remote from the target area.

One method for performing active remote sensing is to stare at an areawith a single detector, while illuminating the area with one or morewavelengths of radiation. Various sources of noise, however, may lowerthe signal-to-noise ratio (SNR) of the measurement. Examples of suchnoise typically present in active remote sensing include solarbackground radiation, 1/f noise (i.e., noise whose power variesinversely with frequency), atmospheric turbulence, and/or scintillation,and noise from varying reflectivity.

Thus, there is a need in the art to perform active remote sensing, usingcompact efficient transmitters and compact receivers while maintaining ahigh signal-to-noise ratio.

SUMMARY OF THE INVENTION

To meet this and other needs, and in view of its purposes, the presentinvention provides a system for measuring properties of a medium. Thesystem includes an electromagnetic generator for forming a CW carrier, adigital encoder for forming a digital message, and a modulator formodulating the CW carrier with the digital message to form a digitallymodulated CW carrier. The medium provides a channel for propagating thedigitally modulated CW carrier. The system also includes a receiverconfigured to receive the propagated, digitally modulated CW carrier,and a processor for measuring at least one property of the medium, orfor identifying the medium. The medium may be disposed within a gaseousatmosphere, a body of water, or a cell of a laboratory.

Another embodiment of the invention is a system for chemicalidentification of a medium. The system includes a laser for generating aCW carrier, a digital encoder for forming an encoded word, and anelectro-optic (EO) modulator for modulating the CW carrier with theencoded word to form an encoded CW carrier. The encoded CW carrier ispropagated through the medium. The system also includes a receiverconfigured to detect the propagated, encoded CW carrier to form adetected signal, and a processor configured to measure bit error rate(BER) of the detected signal and identify the medium based on themeasured BER. The digital encoder may be configured to form at least onepseudonoise (PN) encoded word. The processor may measure the BER of thedetected signal based on the PN encoded word.

Still another embodiment of the invention is a system for chemicalidentification of a medium. The system includes an online laser forgenerating an online CW carrier; an offline laser for generating anoffline CW carrier, the offline CW carrier including a wavelengthdifferent from a wavelength of the online CW carrier; a PN encoder forforming a PN encoded word; a PN′ encoder for forming a PN′ encoded word,the PN′ encoded word being orthogonal to the PN encoded word; andmodulators for, respectively, modulating the online and offline CWcarriers with the PN and PN′ encoded words. The modulated online andoffline CW carriers are propagated through the medium. The systemfurther includes a receiver configured to detect the propagated,modulated online and offline CW carriers to form a detected signal; anda processor configured to correlate the detected signal with the PN andPN′ encoded words, where the processor measures a property of the mediumbased on the correlated detected signal with the PN and PN′ encodedwords. The system may include a wavelength controller, coupled to theonline laser, for modifying a to wavelength of the online CW carrier.The medium may include an absorption line, and the wavelength controllermay be configured to modify the wavelength of the online CW carrier byscanning about the absorption line. The receiver may include a referencedetector for forming a reference signal of the PN and the PN′ encodedwords; and the processor may correlate the reference signal with the PNand the PN′ encoded words, and provide a measure of the effect ofpropagation through the medium, based on (a) correlation of the detectedsignal and (b) correlation of the reference signal, respectively, withthe PN and PN′ encoded words.

Yet another embodiment of the invention is a system for chemicalidentification of a medium. The system includes a laser for generatingan optical beam, a digital modulator for modulating the optical beam toform a digitally modulated optical carrier, and a transmitter fortransmitting the digitally modulated optical carrier through the medium.The system further includes a receiver for detecting the digitallymodulated optical carrier from the medium to form a detected signal, anda processor for measuring at least one property of the medium based onthe detected signal. The receiver may include a first photon counter forcounting photons of the digitally modulated carrier, received from themedium, to form the detected signal. The receiver may also include asecond photon counter for counting photons of the digitally modulatedoptical carrier, transmitted toward the medium, to form a referencesignal. The processor may include a calculator for determining an effectof propagation of the digitally modulated optical carrier through themedium, based on photons counted by both of the photon counters.

It is understood that the foregoing general description and thefollowing detailed description are exemplary, but are not restrictive,of the invention.

BRIEF DESCRIPTION OF THE DRAWING

The invention is best understood from the following detailed descriptionwhen read in connection with the accompanying drawing. Included in thedrawing are the following figures:

FIG. 1 is a block diagram of a digital LIDAR/LADAR system, showing atunable laser transmitter for transmitting a digitally encoded signalthrough optically absorbing gas and a receiver for receiving thedigitally encoded signal and decoding the signal to identify propertiesof the gas, in accordance with an embodiment of the invention;

FIG. 2 is a plot of bit error probability versus E_(b)/N_(o) showingthat the bit error rate (BER) bias may be set by proper choice ofE_(b)/N_(o), in accordance with an embodiment of the invention;

FIG. 3 is a plot of CO2 absorption line as a function of wavelength;

FIG. 4 is a block diagram of a digital LIDAR/LADAR system using adigital communications architecture, including a CW laser transmittertransmitting a wavelength modulated and digitally encoded PN signalthrough a gas absorbing cell, and a digital receiver for decoding thereceived signal using BER as a direct measure of gaseous absorption inthe cell, in accordance with an embodiment of the invention;

FIG. 5 shows a transmitted phase shift keyed (PSK) waveform in which a“1” is represented by a sine waveform and a “0” is represented by a 180°phase-shifted sine waveform, in accordance with an embodiment of theinvention;

FIG. 6 illustrates a sampled, received PSK waveform, based on thetransmitted PSK waveform of FIG. 5, in accordance with an embodiment ofthe invention;

FIG. 7 is a plot of bit error probability versus E_(b)/N_(o) showingthat gas detection sensitivity may be controlled by choosing a digitalencoding method and a modulation type;

FIG. 8 is a block diagram of a digital LIDAR/LADAR system using adigital communications architecture, including a bit error accumulatorfor measuring bit errors between a transmitted digital signal and areceived digital signal, in accordance with an embodiment of theinvention;

FIG. 9 is a block diagram of another embodiment of a digital LIDAR/LADARsystem using a digital communications architecture, including digitalmatched filters for processing the received digital signal, inaccordance with an embodiment of the invention;

FIG. 10 is a more detailed block diagram of the digital matched filtersof FIG. 9, in accordance with an embodiment of the invention;

FIG. 11 is a block diagram of yet another digital LIDAR/LADAR systemusing a digital communications architecture, including two digitallymodulated carriers, the first modulated by a PN encoded signal and thesecond modulated by an orthogonal PN encoded signal (PN′), in accordancewith an embodiment of the invention;

FIG. 12 is a block diagram showing detail of a digital correlationprocessing method that is provided by the system of FIG. 11, inaccordance with an embodiment of the invention;

FIG. 13 is a block diagram showing greater detail of the digital matchedfilters of FIG. 12, in accordance with an embodiment of the invention;

FIG. 14 is a plot of transmittance versus wavelength, showing anexemplary wavelength range used to discretely vary the wavelength of theonline laser of FIG. 11, in accordance with an embodiment of theinvention;

FIG. 15 is a flow block diagram of a BER-based detection method, asexecuted by an embodiment of the present invention;

FIG. 16 is a flow block diagram of a correlation-based detection method,as executed by another embodiment of the present invention;

FIG. 17 is a block diagram of yet another embodiment of a digitalLIDAR/LADAR system using a digital communications architecture,including a photon counting detector, in accordance with an embodimentof the present invention; and

FIG. 18 is a block diagram showing in greater detail two photondetectors and corresponding digital matched filters of the system ofFIG. 17, in accordance with an embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Digital communications is distinctly different from other forms ofcommunications (spark gap, AM, FM, etc). These other forms ofcommunications are fundamentally analog in nature. While FM is superiorto AM, it is, nevertheless, another analog communications method. Analogcommunication methods all share a common feature, in which the goal isto preserve the “shape” of the waveform with as much fidelity aspossible through the communications link. All of the processesassociated with these analog systems pertain to accomplishing the taskof preserving the signal waveform in its entirety, as faithfully aspossible. A difference between AM and FM is the degree of faithfulnessrequired to preserve the signal waveform, in the presence of externalinterference and noise. All of the conventional concepts of linearity,SNR, bandwidth, frequency response, etc., are derived from this overallgoal.

In digital systems, however, the exact shape of the waveform is not thatimportant. The goal of a digital system is to preserve the informationcontent, not the is details of its analog waveform. The informationcontent is represented in binary form and the waveforms need only besufficiently good to preserve the ones and zeros that are of interest.Design of a digital communications system is oriented on preserving theones and zeros and not in faithfully reproducing its analog waveforms.This provides advantages, realized by the inventors, that are notpossible in analog communication systems and may advantageously be usedfor active remote sensing.

For example, an analog repeater is specified in terms of gain, frequencyresponse, linearity and noise. Repeaters that are linear over a largedynamic range are hard to realize. In addition, linear repeaters injectadditional noise which degrades the analog signal after each passthrough a repeater. Consequently, the number of useful repeater stagesin a communications link is limited. This is a basic limitation ofanalog communications.

In digital communications, however, the situation is quite different.Repeaters are replaced with regenerators, in which the ones and zerosare recovered using a binary thresholding process that is immune toanalog noise. In addition, forward error correction (FEC) may be appliedto recover the small number of bad bits that may be received. Theresulting digital data is then re-transmitted as a new stream of onesand zeros at the full signal strength and full data quality of theoriginal message. This process may be repeated practically an unlimitednumber of times with no loss of useful information. This example relatesonly to digital communication systems, as it cannot be done in analogcommunication systems.

While RF and optical communication systems have followed this naturalevolution, active remote sensor technology has not. In fact, activeremote sensing is still seen as a fundamentally analog process, in whichthe goal is still to preserve and measure analog waveforms. Moreover,pulse laser based LIDARs are essentially analogous to spark-gap radiotransmitters. The only way to improve such a system is by building abigger “spark” (i.e. provide more laser power). There is little elsethat may be done, other than minor incremental improvements indetectors, which do not yield significant benefits.

The inventors have discovered that a digital LIDAR or LADAR system maybe used to identify or measure properties of a medium (for example,reflection, shape, polarization, chemical, biological, vibration). Theinventors, thus, use a digital LIDAR or LADAR system as an opticalinformation system rather than as an analog link. The output of thetransmitter of the LIDAR or LADAR system, in one embodiment of theinvention, is a stream of bits that are transmitted into an externalmedium (for example gas) and recovered by a LIDAR or LADAR receiver. Themanner in which the external medium modifies these bits via absorption,polarization effects, reflection, etc., is recovered from the receivedbit sequence by subsequent data processing. This digital “transferfunction” is used to identify or measure the properties of the externalmedium.

Many different digital communication processing techniques may be usedto identify and measure the optical properties of a medium. Thefollowing is a description of some of these processing techniques.

Referring to FIG. 1, there is shown a digital LIDAR/LADAR system,generally designated as 10. System 10 includes wavelength tunable lasertransmitter 12 and digital receiver 16. As shown, the wavelength tunablelaser transmitter transmits an encoded digital signal through anoptically absorbing gas, designated as 14. The same signal is thenreturned to the digital receiver which decodes the encoded signal. Thegaseous absorption of the encoded signal results in a decrease of signallevel or E_(b) (energy per bit), which in turn causes a change in theaverage number of bit errors per second.

As will be explained, in one embodiment of the invention, the bit errorrate (BER) may used as a direct measure of gaseous absorption. Variousmodulation types and various encoding methods may be used to achieve thedesired sensitivity of the transmitted signal to affect the absorbinggas.

The gaseous absorption of the encoded signal, typically, results in adecrease of signal level. Digital receiver 16 measures the BER of thereceived signal at a point where a small change in signal level(E_(b)/N_(o)) results in a large change in bit error probability(P_(B)). This is best illustrated in FIG. 2.

Three curves of bit error probability versus E_(b)/N_(o) are shown inFIG. 2 for coherent phase shift keying (PSK) with different (N,M)encoding schemes. The M is the number of bits in a message and N is thenumber of bits that form an encoded word of the M-message bits.Typically, N is greater than M. The encoding scheme is also known as theBose-Chaudhuri-Hocquenqhem (BCH) Code. One curve, shown in FIG. 2, isfor uncoded (1,1) PSK and the other two curves are for coded (24, 12)PSK and coded (127, 92) PSK. In accordance with an embodiment of theinvention, the BER is set by the digital LIDAR/LADAR system, so that asmall change in received signal level (E_(b)/N_(o)) produces a largechange in bit error probability (P_(B)).

For example, as shown in FIG. 2, the system may select coherent PSK with(24, 12) code and set BER to 1/1000. If the signal processing gain ofthe receiver is set to 6 dB, a very small change in received signallevel would result in a very large change in bit error probability. Asthe transmitter of the LIDAR/LADAR system is tuned in wavelength toidentify, for example, a CO2 absorption line (shown in FIG. 3), thesignal level of the received signal is reduced as a result of thegaseous absorption of CO2. In this manner, CO2 gas may be identified.Other gases may similarly be identified.

As another example, the system may select coherent PSK with (127, 92)code and set the BER to 1/1000. As may be seen in FIG. 2, using the(127, 92) code increases the slope sensitivity of the detection processof system 10. Exemplary detection processes of system 10 using digitalcommunications architecture are discussed below with reference to FIGS.4 through 13.

Referring first to FIG. 4, there is shown a block diagram of a digitalLIDAR/LADAR system, which is implemented using a digital communicationsarchitecture, generally designated as 20. System 20 includes wavelengthcontroller 22, CW laser 26, pseudorandom noise (PN) message encoder 24,optics modulator 28, digital receiver 32 and BER module 34. As shown, PNmessage encoder 24 encodes the transmitted signal, which is used byreceiver 32 to decode the received signal. The transmitted signal ispropagated through (or reflected from) gas absorption cell 30, modifiedor disturbed by the cell, and received by digital receiver 32.

In one implementation, cell 30 may include, for example, a cell in alaboratory environment. In another implementation, cell 30 may include avolume of the atmosphere, which may or may not have a scatteringbackground (e.g., the ground, for a down-looking fielded system, such assystem 10 or 20). Cell 30 may include a solid surface (e.g., theground), objects (e.g., vehicles), vegetation, chemicals, gas/aerosol,or any other typical target of active remote sensing that has spectralfeatures capable of spectral measurement. Cell 30 may include asubstance (for example CO2) having at least one absorption/reflectionfeature, around which the tunable source may be swept in wavelength (CWlaser 26 tuned by wavelength controller 22). Cell 30 may also include asubstance in water (fore example a mineral in the sea).

As will be further described, CW laser 26 is wavelength controlled anddigitally modulated by an encoded PN message (for example). The PNmodulated laser output signal is transmitted through gas absorption cell30. The PN modulated signal is returned and received by digital receiver32, which uses the encoded PN message as a reference to decode the PNmodulated signal. The BER is used as a direct measure for detecting andidentifying the gas absorption cell. As the wavelength of the lasersignal is tuned, the BER measurements detect the absorption line, forexample the CO2 absorption line of FIG. 3. The BER changes very rapidlyat the absorption line.

The encoded PN message, provided by PN encoder 24, may include M messagebits encoded to form an N-bit encoded word. The N-bit encoded word ismodulated onto an optical carrier. In one implementation, anelectro-optic device, such as EO-modulator 28, may be used to modulatethe intensity of a narrow frequency band of a CW laser beam, produced byCW laser 26.

It will be appreciated that digital modulation is a process by whichdigital symbols are transformed into waveforms that are compatible withthe characteristics of a communications channel. Thus, in oneimplementation, the presence of a “1” at the modulator input results inthe transmission of a certain kind of waveform (laser intensity vs time)and the presence of a “0” at the input to the modulator results in thetransmission of another type of waveform. The waveforms for a “1” and a“0” may be represented by a predetermined number of equally-spaced timesamples. These samples may be sent in sequence to the electro-opticdevice to intensity modulate the laser radiation and affect thetransmission of a “1” or a “0” waveform.

By way of example, FIG. 5 shows a transmitted phase shift keyed (PSK)waveform, in which a “1” is represented by a sine waveform (for example)and a “0” is represented by a 180° phase-shifted sine waveform (forexample). The transmitted PSK waveform, as shown in FIG. 5, representsfour bits of “1110”.

Although not shown in FIG. 4, it will be appreciated that EO modulator28 may include an amplifier, such as an Erbium-doped fiber amplifier.The amplified, modulated laser radiation may be radiated into theenvironment using transmit optics to appropriately shape its spatialdistribution and determine its direction of travel. In oneimplementation a collimator to create a slowly diverging beam of laserradiation may be used. The amplified, modulated laser radiation may betransmitted through the medium of interest. The transmitted laserradiation may be intercepted by receiver optics included in digitalreceiver 32.

The received laser radiation may be detected to produce a voltage signalat the input to a demodulator (part of digital receiver 32). Thedetection process may include photo-detection, photocurrent signalamplification, and electronic filtering. The detection process producesan analog signal, which may be sampled, and these signal samples may besent to the demodulator. This is discussed in greater detail withrespect to FIGS. 9, 10, 12 and 13.

By way of example, FIG. 6 illustrates a sampled, received PSK waveform,based on the transmitted PSK waveform of FIG. 5. As shown, there are 20samples per bit. Although during sampling of the received signal,E_(b)/N_(o) is set to only 3.5 dB, receiver 32 recovers the fourtransmitted bits of “1110” of FIG. 5. Recovery of the transmitted PNencoded message is discussed in greater detail later.

Referring next to FIG. 7, another set of curves is shown depicting biterror probability (P_(B)) as a function of E_(b)/N_(o) for variousViterbi and sequential decoding schemes, using coherent binary phaseshift keying (BPSK) over a digital communications channel. As may beobserved (similarly to the set of curves of FIG. 2), as the code lengthis increased (from uncoded, k=1, to a longer code of k=41), thesensitivity of BER may also be increased, by selecting a curve with agreater slope. In this manner, a very small change of E_(b)/N_(o) in thedigital receiver may produce a very large change in bit errorprobability. This change may be measured by BER module 34 of system 20.Thus, gas detection sensitivity of the digital receiver may becontrolled by selecting a proper encoding method and a proper modulationtype.

Referring next to FIG. 8, there is shown a system for measuring opticalproperties of a medium, in accordance with an embodiment of theinvention. The system, generally designated as 40, includes atransmitting portion and a receiving portion. The transmitting portionincludes CW laser 42, modulator 44, optical is amplifier 46 and transmitoptics 48. The receiving portion includes receive optics 52 and detector54. Included in both the receiving portion and the transmitting portionis processor 56. The transmit optics transmits the optical laser signalthrough propagation medium 50. The transmitted laser signal istransmitted through the propagation medium, or reflected from thepropagation medium, and received by receive optics 52.

As shown in FIG. 8, processor 56 includes wavelength controller 58 forcontrolling the output wavelength of CW laser 42. Processor 56 alsoincludes message word module 60, encoder module 62 and encoded messageword module 64. The message word module forms a digital message wordcomposed of M message bits (i.e., a total of M ones and zeros). The Mmessage bits may be message symbols from a larger alphabet. In theexemplary embodiment, however, the message bits are binary and composedof ones and zeros. The collection of M message bits forms the messageword, which may be a randomly distributed set of M ones and zeros. The Mmessage bits are encoded by encoder module 62 to form an N-bit codeword. Typically, N is greater than M, but not necessarily. In oneexemplary embodiment, the encoded message word, output from module 64,may be formed with M=92 and N=127. Another implementation of the encodedmessage word, for example, may be M=12 and N=24, as shown in FIG. 2.

The encoded message word from module 64 is sent to modulator 44, asshown in FIG. 8. The N-bit code word is modulated onto an opticalcarrier provided from CW laser 42. As an example, an electro-opticdevice may be used to modulate the intensity of a narrow frequency bandof a laser beam output from CW laser 42.

By way of modulator 44, the encoded binary message word is transformedinto waveforms that are compatible with characteristics of acommunications channel (for example, the atmosphere). A key requirement,of course, for these waveforms is that they be distinguishable atdigital receive optics 52. Accordingly, in one implementation, thepresence of a “1” at the input of modulator 44 may result in thetransmission of a certain kind of waveform (laser intensity versus time)and the presence of a “0” at the input to modulator 44 may result in thetransmission of another type of waveform.

The waveforms for a “1” and a “0” may be represented by a predeterminednumber of equally spaced time samples. These samples may be sent insequence to the electro-optic device (modulator 44) to intensitymodulate the CW laser radiation and affect the transmission of the “1”and “0” waveforms. Exemplary waveforms for the “1” and the “0” may be asine wave and its 180 degree shifted sine wave. Both waveforms are shownand discussed with reference to FIG. 5.

It will be appreciated that various modulation formats may be used inaccordance with the present invention. For example, binary phase shiftkeying (BPSK), binary frequency shift keying (BFSK), and differentialphase shift keying (DPSK) modulation formats may be utilized. The choiceof modulation format depends on the communications channel through whichthe modulated CW laser beam is intended to propagate. One modulationformat may, of course, be better than another modulation format. Thebest modulation format may be determined by experimentation.

The modulated CW laser signal output by modulator 44 may be amplified byoptical amplifier 46. An exemplary optical amplifier may be anErbium-doped fiber amplifier. Transmit optics 48 may be used to shapethe amplified, modulated laser radiation into a desired spatialdistribution and a desired direction of travel. For example, in oneimplementation a collimator may be used to form a slowly diverging beamof laser radiation.

The amplified, modulated laser radiation is then propagated through themedium of interest, generally designated as 50. The medium may be alaboratory gas cell containing CO2 having a known absorption coefficientfor laser radiation at a center wavelength of the absorption line. Anexemplary center wavelength of an absorption line is shown in FIG. 3 forCO2. Of course, other gases may be used which have different absorptionlines.

The transmitted laser radiation is intercepted by receive optics 52, asshown in FIG. 8. The laser radiation in a fielded-system may, of course,travel through the atmosphere, including propagation medium 50, andencounter a hard target. After the hard target is encountered, the laserradiation is reflected, propagated back through the medium, and returnedto system 40. The received laser beam intensity (or power level)received by receive optics 52 depends on the transmitted laserwavelength and the geometry of the measurement.

The received laser radiation is detected by detector 54, which forms avoltage signal output proportional to the received laser intensity. Asshown in FIG. 8, the output from detector 54 is sent to demodulator 68.The demodulator is shown as residing in processor 56. The detectionprocess performed by detector 54 may include photo-detection,photocurrent, signal amplification, and electronic filtering. Thedetected analog signal is sampled and sent to demodulator 68, as furtherdiscussed with respect to FIGS. 9 and 10.

Demodulator 68 performs the inverse function of modulator 44. Thedemodulator determines, based on the input signal, whether a waveformrepresenting a “1” or a waveform representing a “0” is sent through thepropagation medium. The demodulator makes this determination on a bit bybit basis and requires bit synchronization for successful operation(synchronization between the transmit signal and the receive signal isknown in the art and not shown in FIG. 8).

After the N-bit encoded word is collected in processor 56, the encodedword is decoded into an M-bit received message word by decoder 70. Thedecoder is also shown as residing in processor 56. The detection,demodulation and decoding processes are discussed in more detail withreference to FIGS. 9 and 10.

After decoding of the received message word, the decoded M-bit messageword, from module 72, is sent to bit error accumulator 66. The M-bitmessage word, originally formed by module 60 and transmitted by transmitoptics 48, is also sent to bit error accumulator 66. The bit erroraccumulator compares the received message word with the transmittedmessage word, on a bit by bit basis, and the number of bit errors isdetermined. The bit errors may be accumulated for a predetermined numberof message word transmissions.

After the bit errors have been accumulated, for the predetermined numberof message word transmissions, processor 56, by way of wavelengthcontroller 58, modifies the laser wavelength of CW laser 42 in order totransmit a new CW laser signal. The same message word is encoded bymodule 62 and used to modulate the to new CW laser wavelength by way ofmodulator 44. Again, after receiving and demodulating the new CW lasersignal, the bit errors are accumulated and compared, on a bit by bitbasis, by bit error accumulator 66 for the same predetermined number ofmessage word transmissions.

This process may continue until the end of a desired wavelength scan isreached. When the end of the wavelength scan is reached, processor 56may determine which wavelength produced the highest bit error rate(BER), as discussed previously with respect to FIG. 2. The highest biterror rate (BER) identifies an absorption line of propagation medium 50.

Referring now to FIG. 9, there is shown an exemplary digital receiverused for the BER-based optical detection. The digital receiver,generally designated as 80, shows in greater detail, various processingmodules of processor 56 (FIG. 8). As shown in FIG. 9, the detected datasignal (output from detector 54) includes a signal component and a noisecomponent received by module 82. The detected data signal is sampled ata rate of 1/Ts, where Ts is the sampling duration of thetransmitted/received signal waveforms. As discussed previously, the “1”and “0” waveforms (S1 waveform and S2 waveform, in general) are sent tomodulator 44 to modulate the CW laser signal.

Sampling is performed by switch 84 at a sampling rate of 1/Ts. As shownin FIG. 9, the sampled data signal, r(k), is sent simultaneously todigital matched filter 86 and digital matched filter 88. Assuming thatthe number of samples per each bit is Q, then Q samples may be read intoeach digital matched filter (DMF) 86 and 88. After Q samples are readinto each DMF, the outputs of DMF 86 and DMF 88 may be compared bymodule 90, as shown.

If the output of DMF 86, matched to the S1 waveform, is larger than theoutput of DMF 88, then a “1” is presumed to have been sent/received. Ifthe output of DMF 88, matched to the S2 waveform, is larger than theoutput of DMF 86, however, then a “0” is presumed to have beensent/received. Switch 92 is closed momentarily at a sampling rate of1/T_(b), where T_(b) is bit duration. After each one bit duration ofT_(b) elapses, module 94 forms a 1/0 decision. The 1/0 decision is madeon a bit by bit basis.

Although not shown in FIG. 9, it will be appreciated that digitalreceiver 80 may first achieve signal synchronization bycorrelation/detection of a synchronization word. This synchronizationword may be a preamble, maximal length, PN word that precedes the M-bitmessage word or the N-bit encoded message word. Resynchronization mustbe performed for each change of laser radiation wavelength affected bywavelength controller 58 (FIG. 8). Thus, for each laser radiationwavelength, transmitted through the medium of interest, the waveformsent actually includes a synchronization word that is followed by apredetermined number of message words.

Each 1/0 decision is made after T_(b) bit duration (T_(b)=Q*Ts, where Qis the number of samples per bit and Ts is the sample duration). Thiscontinues until all N-bits have been determined. Recall that there areN-bits for each encoded message word. The encoded message isreconstructed by module 98, on a bit by bit basis, as switch 96 sampleseach bit. After N bits have been reconstructed by module 98, the entireN-bit encoded message is present in module 98.

The N-bit encoded message is decoded by decode module 102 into M messagebits. After decoding by module 102, the M message bits are stored inmodule 106. Switches 100, 104 and 108 are closed momentarily at asampling rate of 1/Tw, where Tw is an encoded word duration, alsodefined as N*Q*Ts. The decoded, received message word is stored inmodule 106. This received message word is compared with the transmittedmessage word of module 110 by error detection module 112. The comparisonmay be performed on a bit-by-bit basis, so that each bit error may becounted.

The process of comparing and detecting bit errors by module 112 may becontinued for a predetermined number of message words. After thepredetermined number of message words have been compared, processor 56(shown in FIG. 8), by way of wavelength controller 58, modifies thewavelength output of CW laser 42. This process may be continued untilthe end of the laser wavelength scan is reached.

It will be appreciated that sampling switches 84, 92, 96, 100, 104 and108 (as well as other sampling switches shown in FIGS. 10, 12, 13 and18) may be digital switches controlled by processor 56, for example.

The digital matched filters (86 and 88) of FIG. 9 are shown in greaterdetail in FIG. 10. As shown, processor 80 includes shift register 126and shift register 128. Each shift register is combined with weightingcoefficients that are matched to input modulation waveforms S₁ or S₂.Weighting coefficients matched to the S₁ waveforms are C₀, C₁, . . .C_(Q−1), designated generally as 130 a, 130 b and 130 n, respectively.Similarly, weighting coefficients matched to the S₂ waveform are C₀, C₁,. . . C_(Q−1), designated generally as 132 a, 132 b and 132 n,respectively.

The contents of shift register 126 and shift register 128 are multipliedby their respective weighting coefficients, and then added together,respectively, by adders 134 and 136 to form separate outputs Z₁(k) andZ₂(k). Separate outputs Z₁(k) and Z₂(k) from digital matched filter 86and digital matched filter 88 are compared after Q samples have beenread. The input to comparator 90 is provided by way of sampling switches138 and 140. The larger output signal from digital matched filter 86 ordigital matched filter 88 is presumed to indicate which waveform wastransmitted. After Q samples have been read, if for example, S₁ wastransmitted, the output of the digital matched filter 86 matched to S₁corresponds to a peak correlation between the received signal and areplica of S₁. If for example, S₂ was transmitted, the output of digitalmatched filter 88 matched to S₂ corresponds to a peak correlationbetween the received signal and a replica of S₂.

It will be appreciated that the signal to noise ratio of a digitalmatched filtered is optimal when matched to a signal passing through alinear system and corrupted by additive white Gaussian noise. FIG. 10may also be better understood by the following set of equationscorresponding to portions of the digital matched filter architecture:

${{SHIFT}\mspace{14mu} {REGISTER}{\mspace{11mu} \;}{LENGTH}} = {Q = \frac{\# {SAMPLES}}{bit}}$r(k) = s(k) + n(k), k = 0, 1, 2, …${{Z(k)} = {\sum\limits_{n = 0}^{Q - 1}{{r\left( {k - n} \right)}C_{n}}}},{k = 0},1,2,\ldots$C_(n) = S_(i)(Q − 1 − n), i = 1, 2.${Z_{i}\left( {Q - 1} \right)} = {\sum\limits_{n = 0}^{Q - 1}{{r\left( {Q - 1 - n} \right)}{S_{i}\left( {Q - 1 - n} \right)}}}$

Another exemplary embodiment of the invention is shown in FIG. 11. Asshown, system 150 provides measurement of optical properties of amedium. System 150 includes a transmitting portion and a receivingportion. The transmitting portion may include wavelength controller 152,online distributed feedback (DFB) laser 156, sync and PN data wordsmodule 154, modulator 158, DFB offline laser 160, modulator 162, syncand PN′ data words module 164, combiner 166, fiber amplifier 168 andcoupler 170. The receiving portion may include science detector 176,reference detector 172, and digital receiver 178. Processor 180 may beincluded in both the receiving portion and the transmitting portion. Theoutput of coupler 170 may be transmitted through propagation medium 174and received by science detector 176.

Wavelength controller 152 may vary the output wavelength of DFB onlinelaser 156 over a range of wavelengths. In one implementation consistentwith the principles of the invention, DFB online laser 156 may beconfigured to scan (or sweep) the output wavelength over a range ofwavelengths, as shown in FIG. 14. That is, DFB online laser 156 maysequentially output discrete wavelengths within the zo sweep range, asshown, during a predetermined period (or inverse of sweep frequency).One exemplary sweep frequency may be approximately 10 hertz, althoughthis is merely an example and other sweep frequencies may be employed.

Wavelength controller 152 may be configured to control the wavelengthproduced by tunable DFB online laser 156, for example, by varying thecurrent that drives tunable laser 156, or by varying the temperature oftunable laser 156. In turn, wavelength controller 152 may receivefeedback signals from tunable online laser 156 to aid in the sweep ofthe laser. The wavelengths of the emitted radiation may fall in theultraviolet, visible, short wavelength infrared (SWIR), mid wavelengthinfrared (MWIR), long wavelength infrared (LWIR), or any other electromagnetic region suitable for active remote sensing. Optics (not shown)may be configured to direct the emitted radiation to modulator 158.

The operation of system 150 may be further described with respect toFIG. 14. The figure is a plot of transmittance versus wavelength for anexemplary sample material having a spectral feature of interest. In someimplementations, the spectral feature of interest may be an absorptionline, such as center absorption wavelength λ_(A). Wavelength controller152 may cause the output wavelength of laser 156 to vary along thewavelength range λ_(SWEEP) at some sweep frequency. The wavelength rangeof λ_(SWEEP) may include the entire spectral feature of interest and mayto extend to wavelengths on either side of the spectral feature ofinterest (or may just extend far enough in wavelength to include thespectral feature of interest). Those skilled in the remote sensing artunderstand how far beyond the spectral region occupied by the spectralfeature of interest the wavelength range of λ_(SWEEP) may extend.

Still referring to FIG. 11, modulator 158 may include an electro-optic(EO) modulator configured to impart modulation to the output of onlinelaser 156. The modulation may be digital message words provided bymodule 154. Continuing the description, DFB offline laser 160 mayprovide a fixed output wavelength, such as λ_(REFERENCE), as shown inFIG. 14. Modulator 162 may modulate the fixed wavelength output fromoffline laser 160 by using digital message words formed by module 164.These digital message words provided by modules 154 and 164 will beexplained in greater detail later.

The online modulated signal and the offline modulated signal may becombined by combiner 166. The combined optical signal may be amplifiedby fiber amplifier 168 and passed through optical coupler 170. A smallfraction of the net signal may be split off to reference detector 172.The reference detector may include a photodetector, amplifier and anelectronic filter. The resulting reference signal may then be routed todigital receiver 178 and processor 180.

The remaining portion of the net optical signal may be radiated throughtransmission optics (not shown) and into propagation medium 174. The netoptical signal traverses the propagation medium, where it may undergoseveral transformations. First, the online and offline portions of thebeam may be amplitude modulated in a random fashion by turbulence, timedependent target reflectivity, speckle effects, vibration misalignmenteffects, and so forth. Because the online and offline wavelengths arenearly identical, these noise effects may be superimposed in nearlyequal fractional gain on the online and offline components of the netpropagating laser beam. Secondly, both the online and offline beams maysuffer the same fractional losses due to radiometric effects (spacelosses due to propagation). Thirdly, and most importantly, the onlinecomponent of the beam may suffer more absorption due to the presence ofa gas of interest, than the offline component of the beam.

The composite laser beam may be collected, after passage throughpropagation medium 174 by receiver optics (not shown) and routed toscience detector 176. The science detector may include a photo-detector,amplifier and electronic filter to produce an analog voltage signalrouted to digital receiver 178. The digital receiver may simultaneouslysample each signal from the science detector and reference detector at arate equal to the sampling rate of the transmitted digital messagewords.

After synchronization is accomplished, by using each sync word frommodules 154 and 164, digital receiver 178 may operate on the sequence ofdata words produced by modules 154 and 164. The processor 180 may alsooperate on the sequence of data words, one word at a time, to extract ameasurement of the relative zo transmission of the online and offlinesignal components of the received laser radiation. These operations arediscussed in greater detail with respect to FIGS. 12 and 13. As will beexplained, the relative transmission may be expressed as a ratio of thescience detector online correlation peak and the science detectoroffline correlation peak, normalized by the same ratio extracted fromthe reference detector channel.

This process may be continued for a predetermined number of data words,after which the wavelength controller may modify the wavelength outputby online laser 156. The process may be continued until the end of thelaser wavelength scan (sweep) is reached. It will be appreciated thatresynchronization, using a sync word provided from module 154 andanother sync word provided from module 164, may be provided for everydiscrete wavelength output by online laser 156.

The digital message words, selected by module 154 and module 164 for theonline and offline laser wavelengths, respectively, will now beexplained. The online digital message may include a preamblesynchronization word, followed by a predetermined number of identicaldata words. The synchronization word and the data words may be maximallength, pseudonoise (PN) code words. Each PN code word may include apredetermined number of bits. The offline digital message may includethe same synchronization word, followed by a predetermined number ofidentical maximum length PN′ code words (PN′ is orthogonal to PN; PN′ isshown in FIGS. 11-13 as PN with a bar on top). The online and offlinemaximal length, code words may be made orthogonal to each other, byforming the offline code word, PN′, from a cyclically shifted version ofthe online code word, PN.

As previously described with respect to FIG. 8, digital modulation byway of modulators 158 and 162 is a process in which digital symbols(1/0) are transformed into waveforms that are compatible with thecharacteristics of a communications channel. As an example, thecommunications channel may be the atmosphere, a channel under water, ora laboratory cell. In one implementation, the presence of a “1” at themodulator input results in a waveform that varies the laser intensity asa function of time, and the presence of a “0” at the modulator inputresults in a transmission of another type of waveform that varies thelaser intensity as a function of time. The waveform for a “1” or a “0”may be represented by a predetermined number of equally spaced timesamples. These samples may be sent in sequence to each modulator tointensity modulate the laser radiation and affect the transmission of a“1” or a “0” waveform.

The digital receiver and processor of FIG. 11 will now be described ingreater detail with reference to FIGS. 12 and 13. Referring first toFIG. 12, system 200 includes science detector 176 and reference detector172. The output from science detector 176 is switched by way of switch202 to digital matched filter 206 and digital matched filter 208.Similarly, the output from reference detector 172 is switched by way ofswitch 204 to digital matched filter 210 and digital matched filter 212.

Digital matched filters 206 and 210 are matched to the PN waveform.Digital matched filters 208 and 212 are matched to the orthogonalwaveform of PN′. The PN is the online signal word and the PN′ is theoffline signal word. PN and PN′ may be orthogonal, maximal length, PNcode words.

As shown in FIG. 12, the detected science and reference data wordsignals may be sampled simultaneously, by way of switches 202 and 204,respectively, at a rate of 1/T_(s), where T_(s) is the sampling durationof the transmitted signal waveform. As previously described, assumingthat the number of samples per bit waveform is Q and the number of bitsper PN data word is P, then Q*P samples are read into each digitalmatched filter (DMF). After the Q*P samples are read into each DMF (206,208, 210 and 212), the output of each DMF is sampled. For the sciencechannel, the output S of the DMF, matched to PN, is a measure of theeffect of propagation through a medium of interest on the online signalcomponent. The output S′ of the DMF, matched to PN′, is a measure of theeffect of propagation through the medium of interest on the offlinesignal component. Similarly, for the reference channel, the output R ofthe DMF, matched to PN, is a measure of the transmitted online signalpower. The output R′ (shown in FIGS. 12 and 13 as R with a bar on top)of the DMF, matched to PN′, is a measure of the transmitted offlinesignal power.

The outputs of S, S′, R and R′ are formed, as shown, after Q*P sampleshave been read into each DMF. The sampling is performed, by way ofswitches 214, 216, 218 and 220. Each of these switches may be closedmomentarily at a sampling rate of 1/Tw, where Tw is the encoded wordduration.

As shown, module 222 forms a ratio of S to S′ (shown in FIGS. 12 and 13as S with a bar on top). Module 224 forms a ratio of R to R′. Module 226forms a normalized power product of each data word that is equal to themeasured value of the relative transmission of the online and offlinelaser beam components. It is noted that the normalized power product isformed by multiplying the output from module 222 with the inverse of theoutput from module 224.

Measuring module 230 may measure the normalized power product providedfrom module 226 by way of sampling switch 228. It will be appreciatedthat the measurement may be performed for each data word, or may beaveraged over a predetermined number of data words to determine a meanrelative atmospheric transmission.

In one implementation, measuring module 230 may process the data to finda single relative transmission value by forming a ratio after summingthe outputs of the DMFs for the predetermined number of data words. Thismay be shown mathematically as follows:

${\tau = {\frac{\sum\limits_{i = 1}^{NDATA}S_{i}}{\sum\limits_{i = 1}^{NDATA}S_{i}^{\prime}} \cdot \frac{\sum\limits_{i = 1}^{NDATA}R_{i}^{\prime}}{\sum\limits_{i = 1}^{NDATA}R_{i}}}},$

where τ is the single relative transmission value, and

-   -   NDATA is the predetermined number of data words processed per        wavelength.

Measurements may be continued for the predetermined number of datawords, after which the wavelength of online laser 156 may be changed.Another measurement may then be taken for the predetermined number ofdata words. This process may be continued until the end of the scan isreached. It will be appreciated that after each change of wavelength,the system may need to be resynchronized by providing new sync words, byway of modules 154 and 164, respectively, of FIG. 11.

The digital matched filters of FIG. 12 are shown in greater detail inFIG. 13. As shown, DMF 206 includes shift register 306 combined withweighting coefficients C₀, C₁ . . . C_(N−1), respectively designated as314 a, 314 b and 314 n. The shift register contents are multiplied bythe weighting coefficients, and added together by is adder 322 to forman output. The length of shift register 306 may be Q*P, where P is thenumber of bits per PN code word, and Q is the number of samples per bit.

The output of adder 322 may be sampled, after an entire PN data word(i.e., Q*P samples) have been read. The sampling may be performed byswitch 214, which may be closed momentarily after the samples for thedata word have been read. The output S from switch 214 may be placed inmodule 338.

In a similar manner, DMF 208 includes shift register 308 combined withweighting coefficients that are designated as 316 a, 316 b and 316 n.The shift register contents are multiplied by these weightingcoefficients, added together by adder 324, and output by way of samplingswitch 216 to form S′ (shown as S with a bar on top) in module 340.Similarly, DMF 210 includes shift register 310 combined with weightingcoefficients that are designated as 318 a, 318 b and 318 n. The outputof shift register 310 are multiplied by these weighting coefficients,and added together by adder 326 to form output R in module 342, by wayof sampling switch 218. Lastly, DMF 212 includes shift register 312combined with weighting coefficients that are designated as 320 a, 320 band 320 n. After multiplication, adder 328, by way of sampling switch220, provides output R′ (shown as R with a bar on top) for placement inmodule 344.

FIG. 13 may also be understood by reference to the following equations:

C _(n) =S _(i)(N−1−n), i=1, 2.

where

S ₁(k)=PN(k), k=0, 1, 2, . . . , N−1

S ₂(k)=PN′(k)

SHIFT REGISTER LENGTH=N

$N = {\left( \frac{\# {SAMPLES}}{BIT} \right){X\left( \frac{\# {BITS}}{{PN}\mspace{14mu} {WORD}} \right)}}$

Measurement Outputs:

$S = {\sum\limits_{n = 0}^{N - 1}{{r_{1}\left( {N - 1 - n} \right)}{{PN}\left( {N - 1 - n} \right)}}}$$S^{\prime} = {\sum\limits_{n = 0}^{N - 1}{{r_{1}\left( {N - 1 - n} \right)}{{PN}^{\prime}\left( {N - 1 - n} \right)}}}$$R = {\sum\limits_{n = 0}^{N - 1}{{r_{2}\left( {N - 1 - n} \right)}{{PN}\left( {N - 1 - n} \right)}}}$$R^{\prime} = {\sum\limits_{n = 0}^{N - 1}{{r_{2}\left( {N - 1 - n} \right)}{{PN}^{\prime}\left( {N - 1 - n} \right)}}}$

It will be appreciated that the matched filter approach shown in FIG. 13using an entire PN code word message may be sufficiently robust toachieve good performance in a turbulent atmosphere. It will further beappreciated that the signal to noise ratio of a digital matched filtermay be optimal when matched to a signal passing through a linear systemand corrupted by additive white Gaussian noise.

Referring now to FIG. 15, there is shown a flow diagram of a BER-baseddetection method. The method, generally designated as 400, will now bedescribed, in connection with previously described FIG. 8. The methodbegins at step 402 and sets a laser radiation wavelength for CW laser42. Step 404, by way of processor 56, forms a message word. The messageword is encoded by encoder 62 in step 406. Next, step 408 modulates theencoded message word from module 64 onto the output of CW laser 42. Themodulated optical carrier is amplified in step 410, by way of opticalamplifier 46.

The method continues to step 412, and radiates the modulated opticalcarrier into the environment (the atmosphere, for example). Theradiation is performed by transmit optics 48 through propagation medium50. Step 414 receives the modulated optical carrier by way of receiveoptics 52, after it is reflected from, or propagated through, themedium.

Method 400 next, by way of step 416, detects the modulated opticalcarrier to form an analog signal. Step 418 next samples the analogsignal. The detection and sampling is performed, for example, by way ofreceive optics 52 and detector 54.

The method then demodulates the sampled analog signal by way of step420. The demodulation may be performed by processor 56 using, forexample, digital matched filters 86 and 88 (FIG. 9). Step 422 decodesthe sampled analog signal to form a decoded message word. This step maybe performed by decoder 70. Performing step 424, method 400 compares thedecoded message word with the transmitted message word on a bit-by-bitbasis. This step may be performed, for example, by bit error accumulator66 upon comparing the received message word placed in module 72 with thetransmitted message word placed in module 60. The bit errors may beaccumulated for a predetermined number of message words in step 426. Aspreviously described, each message word may be identical with any othermessage word during the transmission.

Method 400 branches to step 428 and returns to the start of the method,in order to increment the laser radiation wavelength, by way ofwavelength controller 58. This incremental wavelength may be, forexample, as shown in FIG. 14, one of the discrete wavelengths of λ₁, λ₂,λ₃, etc., up to the end of the wavelength scan designated as λ_(N).Method 400 then repeats steps 402 through 428. If the end of thewavelength scan is reached (for example, λ_(N) has been radiated,received, detected and accumulated as bit errors), then the methodstops.

Referring next to FIG. 16, there is shown a flow diagram of a method fordetecting properties of a medium of interest using a correlation-basedsystem. The method, generally designated as 500, will now be describedin connection with the system previously described with reference toFIG. 11. Method 500 begins at step 502 and sets the online laserradiation wavelength to a predetermined value to form a first opticalsignal. This first optical signal may be, for example, any one of thediscrete wavelengths shown in FIG. 14.

Similarly step 502 sets the offline laser radiation wavelength to apredetermined fixed value to form a second optical signal. The secondoptical signal may be, for example, wavelength λ_(REFERENCE) shown inFIG. 14. As previously described with respect to FIG. 11, the onlinelaser radiation wavelength is set by way of wavelength controller 152and the offline laser radiation wavelength is fixed as a predeterminedvalue in offline laser 160. It will be understood that online laser 156and offline laser 160 each output a CW carrier signal.

Step 504 forms a first PN data message by way of module 154 and forms asecond PN data message (PN′) by way of module 164. Step 506 modulatesthe first and second PN data messages onto the first and second opticalsignals, respectively. These modulations may be performed by modulators158 and 162. Method 500 then, by way of step 508, combines, amplifiesand transmits the modulated first and second optical signals as a netmodulated optical carrier. Step 506 may be implemented by way ofcombiner 166 and fiber amplifier 168.

Step 510 samples the net modulated optical carrier using referencedetector 172, for example, to form a reference analog signal for digitalreceiver 178. Most of the net modulated optical carrier, however, ispropagated into the medium of interest, by way of step 512.

Step 514 receives the net modulated optical carrier. Step 516 detectsand samples the net modulated optical carrier to form a received analogsignal. These steps may be performed by way of science detector 176.Method 500, using step 518, then correlates the received analog signalwith the first and second PN data messages (PN data message and PN′ datamessage) to form the online and the offline received signal components.Method 500 also correlates the reference analog signal with the firstand second PN data messages (PN data message and PN′ data message), instep 520, to form the online and the offline reference signalcomponents. These steps may be performed by way of digital receiver 178and processor 180. The digital receiver samples both the science andreference detector signals simultaneously, at a rate equal to thesampling rate of the transmitted digital messages.

Step 522 then forms a ratio of the online to the offline received signalcomponents, and forms another ratio of the online to the offlinereference signal components. Formation of these ratios is shown, forexample, in FIG. 12, after these ratios have been placed into module 222and module 224. Method 500 next forms a power normalized product, by wayof step 524. An example of a power normalized product is shown in FIG.12, after this product has been placed into module 226.

In one implementation, as shown in FIG. 16, methods 500 averages thepower normalized product over a predetermined number of data messageword, by way of step 526. The method, by way of step 528, returns to thestart and increments the online laser radiation wavelength, aspreviously described. If the end of the wavelength scan is reached,method 500 stops.

As previously described, the online laser radiation wavelength is set tosome desired starting value, for example, λ₁ of FIG. 14. This wavelengthmay be discreetly swept across an absorption line of interest. Suchwavelengths of interest are shown, for example, as λ₁, λ₂, etc., up tothe end of scan of λ_(N). One cycle of steps 502 through 528 may beperformed for each discrete wavelength in the scan.

The aforementioned figures have shown various approaches for detecting,identifying, and measuring optical properties of a medium of interestusing communication architectures. These communication architectures(for example system 10, system 20, system 40, and system 150) may beconfigured using lasers that generate continuous laser beams, or CWoptical carriers. These CW beams, reflected from the medium of interest,may be detected by each of these architectures. Because of the digitalprocessing, as exemplified by these architectures, the returned beamadvantageously provides a much needed signal-to-noise ratio. Moreover,small, low power, light weight, continuous fiber lasers may be used. Thesystem optics are advantageously simple, cost effective, and noncritical. Furthermore, the receiver and transmitter may be configuredwithin a small integrated system.

The online/offline ratio, implemented by system 150 (FIG. 11), is notdominated by turbulence or speckle noise. As known by those skilled inthe art, air turbulence causes noise. The density of the gases and theatmosphere at any given location changes as a function of temperature,pressure variation, wind velocity, eddies and shear caused by winds.Turbulence causes changes in the index of refraction of light of theair. Such changes of the index of refraction causes interference effectsthat change as a function of time. These changes are known as speckleeffects, or speckle twinkle.

Speckle may also occur as coherent light (laser light) reflecting off asurface. The speckle pattern is determined by the surface roughness andthe transmitter diameter. Surface reflecting zones reflect back thelaser light with different optical path links, causing zones of lightand dark at the received aperture. A moving sensor on an aircraft, forexample, may cause these zones of light and dark to change. These lightvariations manifest themselves as noise superimposed on the incomingreceived signal. The inventors have discovered, however, that using theaforementioned online/offline ratio method, as implemented by system 150for example, mitigates noise due to turbulence effects.

Examples of embodiments providing correlation detection architectureshave been described with respect to FIGS. 11 and 12. Another example ofa correlation detection architecture will now be described with respectto FIGS. 17 and 18. Referring first to FIG. 17, there is shown system600 providing measurement of optical properties of a medium. System 600includes a transmitting portion and a receiving portion. Thetransmitting portion includes components that may be similar tocomponents of the transmitting portion of system 150 (FIG. 11).Accordingly, the transmitting portion may include wavelength controller152, online distributed feedback (DFB) laser 156, sync and PN data wordsmodule 154, modulator 158, DFB offline laser 160, modulator 162, syncand PN′ data words module 164, combiner 166, fiber amplifier 168 andcoupler 170.

The receiving portion of system 600 may include a photon countingscience detector 602, reference detector 604, digital receiver 178 andprocessor 180. Processor 180 may be included in both the receivingportion and the transmitting portion of system 600. The output ofcoupler 170 may be transmitted through propagation medium 174 andreceived by photon counting science detector 602.

The photon counting science detector 602 replaces science detector 176of system 150 (FIG. 11). It will be appreciated that science detector176 may include an analog photo detector, an amplifier/filter, and ananalog/digital converter (which provides analog samples to digitalreceiver 178). Photon counting science detector 602 may replace thesethree components, namely the analog photo detector, amplifier/filter,and analog/digital converter.

The photon counting science detector 602 may include a photon countingdetector, a count accumulator, and a reset control for resetting thecount accumulator, after the count accumulator is read. These are shownin FIG. 18, for example, as photon counting detector 702 and countaccumulator 704. Although not shown, the reset control may be providedby way of processor 180.

The sequence of output values provided from the count accumulator for apredetermined received waveform may be called a count vector.Accordingly, instead of correlating on analog sample values, asdescribed with respect to system 150 (FIG. 11), photon counting sciencedetector 602 may establish a count vector for the PN codeword. Thecorrelation may then be performed using the count vector information.

The composite laser beam collected by receiver optics (not shown), afterpassage through propagation medium 174, may be routed to photon countingscience detector 602. Although the photon counting science detector mayreceive a very weak return signal, nevertheless the photon countingscience detector may advantageously detect the photonic nature of thereturn signal, by way of photon counting detector 702 and countaccumulator 704. As a result, a discrete photoelectron count signal maybe routed to digital receiver 178.

It will be understood that reference detector 604 may include an analogdetector that is similar to the reference detector of system 150 (FIG.11). As an alternate embodiment, reference detector 604 may includecomponents that are similar to photon counting science detector 602. Asshown in FIG. 18, reference detector 604 includes photon countingdetector 706 and count accumulator 708.

Still referring to FIG. 18, system 700 may include components that aresimilar to components shown in the correlation architecture of system200 (FIG. 12), with the exception of photon counting science detector602 and photon counting reference detector 604, both shown on the leftportion of FIG. 18.

Digital receiver 178 (FIG. 17) may simultaneously sample both signalsfrom science detector 602 and reference detector 604, at a rate equal tothe sampling rate of the transmitted digital message. Aftersynchronization is accomplished (as previously described), digitalreceiver 178 and processor 180 may process the sequence of data words,one word at a time, to extract a measurement of the relativetransmission of the online and offline signal components of the receivedlaser radiation. The correlation of both science and reference detectoroutputs with DMFs matched to PN and PN′ (as shown in FIG. 18) allows apower normalized measurement of the relative transmission of the onlineand offline signal components of the received laser radiation.

The relative transmission may be given by the ratio of the sciencedetector online correlation peak to the science detector offlinecorrelation peak normalized by the same ratio extracted from thereference detector channel. This process may be continued for apredetermined number of data words, after which (as explainedpreviously) system 600, by way of wavelength controller 152, may modifythe wavelength of online laser 156. The relative transmission may againbe measured, and then the wavelength of online laser 156 may again bemodified. When the end of the laser wavelength scan is reached, theprocess may be stopped.

As previously described, signal synchronization may be accomplished bycorrelation detection of a preamble, maximal length, pseudo noise (PN)synchronization word. Resynchronization may be performed at each changeof laser radiation wavelength. Accordingly, for each interrogatingonline laser radiation wavelength, the waveform sent through the systemmay include a preamble synchronization word followed by a predeterminednumber of online and offline data words.

The received signal may be sensed in the science channel by photoncounting detector 702 and accumulated by count accumulator 704. Thecounts may be accumulated at a sample duration of T_(s) and reset aftereach interval. The reference signal in the reference channel may bedetected and accumulated in a manner similar to the detection andaccumulation of the science signal, or may be sampled using an analogdetector, as previously described.

Assuming that the number of samples per bit waveform is Q, and thenumber of bits per PN data word (or PN′ data word) is P, then Q*P countvalues may be read into the science channel digital matched filters(DMFs 206 and 208), and Q*P count values or samples (depending on thereference detection method) may be read into the reference channeldigital matched filters (DMFs 210 and 212).

The DMFs shown in FIG. 18 may be similar to the DMFs shown in FIG. 13.The output signals of S, S′, R, and R′ shown in FIG. 18 may be formedsimilarly as previously described with respect to FIGS. 12 and 13. Theratios formed by modules 222, 224 and 226 may be similar to the ratiosdescribed with respect to FIGS. 12 and 13. Although not shown, it willbe appreciated that these modules may reside in processor 180. Finally,the measurements performed by module 230, which may also be part ofprocessor 180, may be similar to those described previously with respectto FIG. 12.

Although the invention is illustrated and described herein withreference to specific embodiments, the invention is not intended to belimited to the details shown. Rather, various modifications may be madein the details within the scope and is range of equivalents of theclaims and without departing from the invention. For example, anydigital processing system, known in the field of communications, may beused as an embodiment of the present invention for processing adigitally transmitted/received optical signal to detect and measure aproperty of a medium. Any communications architecture that preserves theinformation content of a digitally transmitted signal may be used as anembodiment of the present invention, because digital processingtechniques may advantageously provide better signal-to-noise of areceived signal, as compared to a received signal processed in an analogcommunications system.

Extensions and generalizations that use Geiger-mode detection andstatistical data processing to extract information may also be used bythe invention. These may be used for molecular backscatter measurementsand for very low power trace gas detection.

The invention may be used in various weather and climates and may beused to detect, for example, pollution, bio-hazards, and weapons of massdestruction. The invention may also be used in space, on an airborneplatform or a ground platform. The platform may be mobile or stationary.The invention may also be used under water and may be a handheld system.

1. A system for chemical identification of a medium comprising: anonline laser for generating an online CW carrier, an offline laser forgenerating an offline CW carrier, the offline CW carrier including awavelength different from a wavelength of the online CW carrier, a PNencoder for forming a PN encoded word, a PN′ encoder for forming a PN′encoded word, the PN′ encoded word being orthogonal to the PN encodedword, modulators for, respectively, modulating the online and offline CWcarriers with the PN and PN′ encoded words, a medium for propagating themodulated online and offline CW carriers, a receiver configured todetect the propagated, modulated online and offline CW carriers to forma detected signal, a processor configured to correlate the detectedsignal with the PN and PN′ encoded words, and the processor configuredto measure a property of the medium based on the correlated detectedsignal with the PN and PN′ encoded words.
 2. The system of claim 1wherein the medium is disposed within a gaseous atmosphere, a body ofwater, or a cell of a laboratory.
 3. The system of claim 1 including awavelength controller, coupled to the online laser, for modifying awavelength of the online CW carrier, and wherein the medium includes anabsorption line, and the wavelength controller is configured to modifythe wavelength of the online CW carrier by scanning about the absorptionline.
 4. The system of claim 1 wherein the receiver includes a referencedetector for forming a reference signal of the PN and the PN′ encodedwords, the processor is configured to correlate the reference signalwith the PN and the PN′ encoded words, and the processor is configuredto provide a measure of the effect of propagation through the medium,based on (a) correlation of the detected signal and (b) correlation ofthe reference signal, respectively, with the PN and PN′ encoded words.5. A system for chemical identification of a medium comprising: a laserfor generating an optical beam, a digital modulator for modulating theoptical beam to form a digitally modulated optical carrier, atransmitter for transmitting the digitally modulated optical carrierthrough the medium, a receiver for detecting the digitally modulatedoptical carrier from the medium to form a detected signal, and aprocessor for measuring at least one property of the medium based on thedetected signal, wherein the processor includes code generators forforming a first encoded word and a second encoded word, the laserincludes an online laser and an offline laser for generating,respectively, an online beam and an offline beam, the digital modulatorincludes a first modulator for modulating the online beam using thefirst encoded word, and a second modulator for modulating the offlinebeam using the second encoded word, a combiner for combining themodulated online and offline beams to form the digitally modulatedoptical carrier, and the processor including a digital matched filterfor correlating the detected signal with the first and second encodedwords.
 6. The system of claim 5 wherein the receiver includes a photoncounter for counting photons of the digitally modulated optical carrier,received from the medium, to form the detected signal.
 7. The system ofclaim 6 wherein the receiver includes another photon counter forcounting photons of the digitally modulated optical carrier, transmittedtoward the medium, to form a reference signal, and the processorincludes a calculator for determining an effect of propagation of thedigitally modulated optical carrier through the medium, based on photonscounted by both of the photon counters.
 8. A method for chemicallyidentifying a medium comprising: (a) generating an online CW carrier;(b) generating an offline CW carrier, the offline CW carrier including awavelength different from a wavelength of the online CW carrier; (c)forming a PN encoded word; (d) forming a PN′ encoded word, the PN′encoded word being orthogonal to the PN encoded word; (e) modulating theonline and offline CW carriers with the PN and PN′ encoded words,respectively; (f) propagating the modulated online and offline CWcarriers into a medium; (g) receiving the propagated, modulated onlineand offline CW carriers to form a detected signal; (h) correlating thedetected signal with the PN and PN′ encoded words; and (i) measuring aproperty of the medium, based on the correlated detected signal with thePN and PN′ encoded words.
 9. The method of claim 8 including modifying awavelength of the online CW carrier; and controlling the wavelength ofthe online CW carrier to include an absorption line of the medium. 10.The method of claim 8 including forming a reference signal of the PN andthe PN′ encoded words; and wherein step (h) includes correlating thereference signal with the PN and the PN′ encoded words; and step (i)includes measuring an effect of propagation through the medium, based on(1) correlation of the detected signal and (2) correlation of thereference signal, respectively, with the PN and PN′ encoded words.
 11. Amethod for chemically identifying a medium comprising: (a) generating anoptical beam; (b) modulating the optical beam to form a digitallymodulated optical carrier; (c) transmitting the digitally modulatedoptical carrier through the medium; (d) receiving the digitallymodulated optical carrier from the medium to form a detected signal; and(e) measuring at least one property of the medium based on the detectedsignal, wherein step (a) includes generating an online beam and anoffline beam; step (b) includes forming a first encoded word and asecond encoded word, modulating the online beam using the first encodedword, and modulating the offline beam using the second encoded word; andcombining the modulated online and offline beams to form the digitallymodulated optical carrier; and step (d) includes correlating thedetected signal with the first and second encoded words.
 12. The methodof claim 11 wherein step (d) includes counting photons of the digitallymodulated optical carrier, received from the medium, to form thedetected signal.
 13. The method of claim 12 wherein step (d) includescounting photons of the digitally modulated optical carrier, transmittedtoward the medium, to form a reference signal; and step (e) includesdetermining an effect of propagation of the digitally modulated opticalcarrier through the medium, based on photons counted in step (d).